use settings.EMAIL_NIGHTLY_REPORTS to send (#2166)

By default generate nightly reports don't send.  Set to True in settings.py or
database.py to re-enable.
support generating reports locally, not s3 (#2166)

Doesn't completely work, but this helps get us there.
nightly graphs: use main readonly db config (cleanup)
fix condition on creation readonly db stanza

My ruby isn't great.  I noticed in a dev environment (where a readonly
db isn't present) that I was creating the readonly stanza, but it was
empty.  This fixes that.
fix quoting bug in EMAIL_NIGHTLY_REPORTS (#2166)
Merge branch 'sef-2166-stop-nightly-emails' of github.com:Stanford-Online/class2go into sef-2166-stop-nightly-emails
Merge pull request #2178 from Stanford-Online/sef-2166-stop-nightly-emails

Operational things for sprint 33